This invention relates to a black colorant composition which appears to have a substantially constant color, regardless of the light source. The colorant composition may be incorporated into a resin.
A material will appear black if substantially all of the light in the visible electromagnetic spectrum (400-700 nanometer) is absorbed. Thus, black colorant compositions may be mixtures of two, three or more individual colorants, which compliment each other to absorb light across the spectrum. For example, orange and blue; yellow, red and blue; and orange, blue and purple represent color combinations which will create a black composition.
In general, a black colorant composition made by combining two or more colorants will absorb electromagnetic radiation across the entire visible spectrum, but the absorbance at each wavelength may not be uniform. Consequently, the absorbance of such a composition may be represented by a series of peaks and valleys across the visible spectrum. The non-uniformity of the absorbance can result in the composition shifting color when the light source is changed.
One of the characteristics of a colorant composition is its color constancy or conversely, its tendency to change hue depending on the light source, referred to as flair or flairing. This change in appearance based on the light source is primarily due to the difference in spectral distribution between the light sources. The more energy emitted at a given wavelength, the more energy that must be absorbed by the individual colorants in the composition. Otherwise, light from that particular wavelength will be reflected by the substrate making the composition look different than when viewed under a light source that does not have the same spectral power distribution.
The tendency of some compositions to flair can be understood when one considers the difference in spectral power distribution between the illuminants "D.sub.65 ", representing daylight, and "A", representing a tungsten light source, such as the normal incandescent light bulb. In color terminology D.sub.65 would have a black body temperature of 6,504 degrees Kelvin while the "A" light would be described as having a black body temperature of 2,856 K. The relative power distributions from these two light sources at the various wavelengths can be see in FIG. 1. Any light that is not absorbed is reflected by the substrate, causing the substrate to appear whatever shade the reflected wavelength would provide. For instance reflected light in the 400-430 nanometer range would appear violet. Similarly, reflectance in the ranges 430-480, 480-560, 560-590, 590-620, and 62-700 would appear blue, green, yellow, orange, and red respectively.
Flairing is observed in both individual colorants containing a single chromophore and colorant compositions. It has generally been believed that in order for a colorant composition to have low flairing, the individual colorant components should exhibit low flairing. Carbon black has been considered the standard for color constancy/low flairing for black colorants.
Colorants are commonly used in the manufacture of polyurethane foam. Poly(oxyalkylene) substituted colorants have been found to be particularly useful in that they may react with the polyisocyanate monomers and become permanently bound in the resin. Examples of such colorants may be found in the following United States Patents:
U.S. Pat. No. Cross et al. 4,284,729 PA1 U.S. Pat. No. Kluger et al. 4,507,407 PA1 U.S. Pat. No. Kluger et al. 4,751,254 PA1 U.S. Pat. No. Kluger et al. 4,761,502 PA1 U.S. Pat. No. Kluger, et al. 4,775,748 PA1 U.S. Pat. No. Rekers et al. 4,846,846 PA1 U.S. Pat. No. Kluger et al. 4,912,203 and PA1 U.S. Pat. No. Kluger et al. 4,978,362
Note U.S. Pat No. 4,751,254, Example 7, which reports that certain benzathiozole azo colorants are not stable in the presence of stannous octoate catalyst used in polyurethane foam production.